1. Field of the Invention
The present invention relates to a power supply on/off switching means and, in particular, to a power supply switching means offering inrush current limiting and increased reliability.
2. Description of the Prior Art
A recurring problem of digital data processing systems is that of power supply failure resulting in a loss of data, including system operating state. At best, such a failure may require the reloading of data and programs and, at worst, may result in an irretrievable data loss.
The two major sources of system power supply failure are the power supply itself and the means for controlling the turn-on and turn-off of the power supply. In general, failures of the power supply may be guarded against by the provision of alternate system power sources, such as battery back-up systems, which can provide power to a system for the period necessary to close the system down without data loss.
The second problem, failure of the power supply on/off control means, is in certain respects more difficult to guard against in that many such failures occur while the system power is in a dynamic, transient state, that is, during turn-on or turn-off.
Should a failure occur during this transient state, a battery back-up system may not be able to prevent the loss of data. As such, it is preferable for a power system to fail while it is in a stable state, that is, either to fail to turn on or to fail while on rather than failing during turn-on or turn-off.
The two principle sources of power supply control failure are, first, the normal, expected failure of components and, second, the failure of components due to increased stress during system turn-on and turn-off. As an example of the latter, current surge occurring during turn-on of a power supply can, for example, cause power supply rectifiers to fail. In the prior art, inrush current has customarily been limited by the use of current limiting elements in the power input line, such as thermistors and resistors switched by triacs. The disadvantages of such devices are that thermistors remain in the power line, thereby dissipating power, while switching triacs are active devices and are required to carry the full input current to the power system during the entire time the system is on.